Method for coupling a laser beam into a liquid-jet

Abstract

Reliable coupling of a high-power laser beam into a liquid-jet in a liquid-jet guided laser system can be achieved with high lifetime performance of the nozzle and the protection window, through setting the parameters of the liquid-jet guided laser system according to an optimum relationship that links the focus point of the laser, the focus cone angle, the laser beam energy distribution profile and the nozzle geometry.

What is claimed is: 
     
       1. A method comprising:
 providing a liquid guided laser system, wherein the liquid guided laser system comprises:
 a nozzle with a nozzle inlet for forming a liquid jet, 
 a laser beam and a lens for focusing the laser beam, wherein the laser beam passes through the lens, and then through the nozzle inlet, wherein the liquid jet is configured to guide the laser beam; 
 
 determining a laser power density profile; 
 calculating a focus point for the laser beam based on a relationship between the laser power density profile and the nozzle inlet diameter; 
 setting a focus of the laser beam based on the calculated focus point by adjusting the lens, wherein the adjusting of the lens is such that the lens is moveable in an upward and downward direction in relation to a nozzle inlet plane, such that the focus of the laser beam is adjusted to be below the nozzle inlet plane. 
 
     
     
       2. A method as in  claim 1 , further comprising:
 wherein the relationship is based on the laser power density profile wherein a peak output of the laser beam profile passes through a radial center of the nozzle, wherein a minimum output of the laser beam hits a sidewall of the nozzle as the laser beam passes through the nozzle. 
 
     
     
       3. A method as in  claim 2 , further comprising:
 wherein the minimum output of the laser beam hitting the sidewall of the is less than 1% or less than 1 W of the laser power density profile. 
 
     
     
       4. A method as in  claim 1 , further comprising:
 wherein the relationship comprises a coupling factor, wherein the coupling factor represents in a function the nozzle inlet diameter as a whole in relation to a standard beam profile having a 1/e 2  diameter. 
 
     
     
       5. A method as in  claim 4 , further comprising:
 wherein the coupling factor is selected so that a portion of the laser power density profile hitting the nozzle is less than 1% or less than 1 W. 
 
     
     
       6. A method as in  claim 1 , further comprising:
 wherein the relationship comprises a diameter of the nozzle, numerical aperture of the lens, and index of refraction of the liquid at a wavelength of the laser beam. 
 
     
     
       7. A method as in  claim 1 , further comprising:
 wherein the relationship is proportional to 
 
       
         
           
             
               
                 
                   0.5 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     D 
                     N 
                   
                 
                 
                   tan 
                   ⁡ 
                   
                     ( 
                     
                       arcsin 
                       ⁡ 
                       
                         ( 
                         
                           NA 
                           / 
                           n 
                         
                         ) 
                       
                     
                     ) 
                   
                 
               
               , 
             
           
         
         
           wherein D N  is a diameter of the nozzle, 
           wherein NA is a numerical aperture of the lens, and 
           n is an index of refraction of the liquid at a wavelength of the laser beam. 
         
       
     
     
       8. A method as in  claim 1 , further comprising:
 wherein setting the focus of the laser beam based on the calculated focus point comprises setting the focus to be between 0.8 and 1.2 times the diameter of the nozzle inlet. 
 
     
     
       9. A method as in  claim 1 , further comprising:
 selecting an internal reflection of the laser beam in the liquid jet to be at a laminar flow portion of the liquid jet.